If a system objects is isolated from its environment and does not interact with its environment at all, then certain physical quantities of the system cannot change. They are constant or conserved. Conservation laws are considered to be the most fundamental principles of physics.
In mechanics. energy, momentum, and angular momentum are conserved for an isolated system. There are, however, no truly isolated systems. Interactions with the environment can only approximately excluded. So while conservation laws are very useful in analyzing many everyday situations, they strictly hold only for the universe as a whole.